The invention relates to a method and an apparatus for transmitting data.
xDSL methods (DSL: Digital Subscriber Line) are transmission-related methods for the digital use of twisted pairs of telephone wires in a subscriber access network. The subscriber access network is a network to which a multiplicity of subscribers are connected by means of a respective telephone line. In the case of xDSL, a distinction is drawn between symmetrical and asymmetrical xDSL variants. In the case of the asymmetrical ADSL, the data transmission rates downstream, i.e. towards the subscriber, at up to 8 MB/sec., are much higher than the data transmission rate of 1 MB/sec. which can be achieved upstream. In most households, Internet access via the existing copper pairs is used. The continually rising need for bandwidth also means that what is known as VDSL (Very High Speed Digital Subscriber Line, ITU-T Standard G.993.1 G.993.2) technology is also being introduced in steps, this technology allowing data rates of up to 100 MB/sec. to be achieved in both directions on short lines.
The attainable data rates are dependent on the signal-to-noise ratio SNR on the respective telephone line. As the line length increases and as the crosstalk from other telephone line pairs increases, the data rate which can actually be attained decreases. The current practice of a network operator for configuration during operation of a DSL line is for the customer or subscriber to have a fixed data rate agreed for his Internet access which is significantly below the physically achievable data transmission rate. This agreed low fixed data transmission rate is also still achieved on longer telephone lines, even if disturbing adjacent DSL signals occur. Alternatively, network operators guarantee a minimum data transmission rate and then set a possibly higher possible data transmission rate on the basis of the actual line conditions when clearing the DSL line (Best Effort).
It is becoming increasingly important for DSL network operators to provide voice services (Voice Over IP) and films (Video On Demand) using DSL in future. This significantly increases the need for bandwidth. Transmitting voice data using Voice Over IP and transmitting image data make special demands on the DSL connection. Voice data have a low data transmission rate of below 100 KB/sec. but are particularly sensitive to a long signal propagation time or signal propagation time fluctuations, since these mean that signal components reflected at the far end of the subscriber line are perceived by the subscribers as an irritating echo. Merely a total propagation time of above 30 msec. reduces the subjective quality of the telephone call. A DSL voice link should therefore ideally not contribute more than 5 msec. to the total propagation time.
Video or image data use image compression techniques which result in a data transmission rate of between 2 MB/sec. and 20 MB/sec. The real-time character of the video data stream does not allow the repeated transmission of data packets which have been lost, as is customary with the TCP protocol, for example (TCP Over IP). If RTP data packets (RTP: Real Time Protocol) are lost, this results in brief picture noise. Retransmission of lost data packets for image data transmission is possible only if a sufficiently large buffer store is provided at the receiver end which can compensate for the time loss in the event of a fresh packet request. However, a large buffer store has the drawback that a change of channel or a change of program results in long idle times when emptying or refilling the buffer store, or else a very large bandwidth needs to be provided on the DSL line which is significantly higher than the ascertained data rate required for transmitting a single video data stream.
Impulse noise occurring on a DSL connection can result in a brief loss of data. For pure Internet data, such as web data, or e-mail data, such data losses caused by impulse noise are totally unnoticeable to the subscriber thanks to TCP retransmission, i.e. fresh transmission of data. Even when voice data are being transmitted, impulse noise merely causes an audible noise, but this is barely perceived by the subscribers.
Both ADSL and VDSL are equipped with an error correction mechanism where the transmitter adds redundant data to the data stream which allow a particular volume of noisy data to be reconstructed at the reception end. This error correction allows bit errors caused by impulse noise to be corrected. In this case, bit errors when pulsed noise signals occur are avoided by distributing the bit information over the DSL connection in the course of time, or “interleaving” it. Without interleaving, a pulsed noise signal can disturb both the actual useful data and the redundantly added error correction data. However, interleaving the data results in an additional signal propagation time which is typically between 80 and 20 msec. in order to provide adequate protection against pulsed noise signals. Pulsed noise signals normally have a duration of less than 0.5 msec. During the occurrence of a pulsed noise signal, the data contained in the data stream are completely destroyed. The longer signal propagation times caused by interleaving can be accepted without any drawbacks for video or image data. By contrast, longer signal propagation times caused by interleaving have a disruptive effect when transmitting voice data and possibly also when transmitting Internet traffic where timing is critical, for example “online gambling”.
Generally, the various traffic types of data, particularly Internet data, voice data and video data, place partly conflicting demands on the configuration of the DSL connection and hence of the xDSL modem.